The present invention relates to hub assemblies, and particularly to hub assemblies for bicycle wheels.
Lightweight bicycle components improve the efficiency and performance of the bicycle to which they are fitted. In many cycling activities, and particularly in competition racing, even small reductions in the weight of components are regarded as highly significant. One part of the bicycle for which it is desirable to produce lightweight components, while maintaining sufficient strength and reliability, is the bicycle drive system, which includes the rear wheel hub, the chain, the derailleur, the chain rings which drive the chain, and the sprockets which are attached to the hub and driven by the chain.
In order to reduce the weight of the drive system it is desirable to use sprockets which are as small as possible. Smaller sprockets are of course lighter, but they also produce higher gear ratios for any given size of chain ring, and this may also enable the size and weight of the chain rings to be reduced. Because the sprockets are of smaller diameter than the chain rings, any reduction in the number of teeth on a sprocket produces a greater increase in gear ratio than would a corresponding reduction in the number of teeth on a chain ring. This means that significant reductions in size of the chain rings can be achieved for relatively modest reductions in size of the sprockets, while maintaining the same effective range of gear ratios.
Furthermore, the reduction in size of the sprockets and chain rings enables a shorter, and hence lighter, chain to be used and also makes it possible to reduce the size of derailleur which is required. Alternatively, if no reduction in the weight of the drive system is required, small sprockets can be used simply to increase the maximum gear ratio of the bicycle. It will be seen therefore that a large number of benefits can arise from sprockets having a small number of teeth. However, it has hitherto proved to be difficult to use a sprocket having less than twelve teeth with conventional bicycle hub assemblies.
One reason for this difficulty arises in the case where a free-wheel is required. In this case the sprockets are non-rotatably mounted on a sprocket carrier which is mounted on a sprocket carrier support by ball bearings, or sometimes needle bearings, to allow the carrier and sprockets to rotate relatively to the sprocket carrier support. A one-directional coupling, such as a pawl and ratchet device, is connected between the carrier and support to permit relative rotation between the sprockets and support in only one direction. The necessity of providing ball or needle bearings, and appropriate mountings for such bearings, between the carrier and carrier support means that, in practice, there is a minimum practical outer diameter for the sprocket carrier and consequently a minimum size for the smallest sprocket.
In one conventional type of hub assembly, the sprocket carrier support is formed in two separable parts: a main part on which the sprocket carrier is rotatably mounted, and an intermediate part which is integrally formed at one end of the main hub unit. These two parts of the support are formed with interengaging screw threads so that the main part, bearing the sprocket carrier and sprockets, may be attached to the hub unit by screwing it on to the intermediate part. When assembling the hub unit, therefore, it is necessary to pass one of the ball-bearing sets through the intermediate part of the sprocket carrier support. This imposes a minimum diameter on the intermediate support part and this may, in turn, impose a minimum diameter on the size of sprockets which can be used on the assembly.
In another known arrangement, the sprocket carrier support is omitted, and ball-bearing sets are located directly between the sprocket carrier and the axle. Again, however, the necessity of providing such ball-bearing sets imposes a minimum diameter on the sprocket carrier, and hence on the sprockets.